Many manufacturing and/or assembly processes (generically production processes) require that structural elements of similar or dissimilar materials be secured in combination. One method employed in such production processes to secure structural elements in combination is to provide an adhesive bond between the structural elements. One critical feature of such production processes is the nature, i.e., characteristics or properties, of the adhesive system (film or paste adhesives) that provides the bonding force. Characterization testing for adhesive systems generally involves applying an adhesive system between overlapping structural elements, and curing the adhered, overlapped structural elements to form a bonded test specimen which may then be subjected to various mechanical and/or chemical tests.
Industry performs characterization testing of adhesive systems to verify, establish and/or define the properties of adhesive systems, e.g., shear strength, cure temperature, cure pressure, which make various adhesive systems effective bonding agents in different production processes. Characterization databases for adhesive systems are continually being updated and/or expanded to provide industry with current knowledge regarding the properties of various adhesive systems. In the aerospace industry alone, over one hundred different adhesive systems are available as bonding agents for production processes. Moreover, approximately twelve new adhesive systems are developed each year for bonding applications in aerospace production processes.
In performing characterization testing o adhesive systems to develop property databases for adhesive systems, it is extremely important that the testing procedure produce both standardized and reproducible test results for comparative purposes, i.e., the properties of the adhesive systems are not subjected to test specimen and/or fixture induced variabilities. Standardization ensures that evaluation and selection of an adhesive system for a specific production process application is based upon a logical and reliable foundation.
A number of factors are involved in ensuring that standardized and reproducible test results are achieved in characterization testing of adhesive systems. The first and foremost factor to be considered is the test specimen bonding site, i.e., the bonding area to which the adhesive system to be tested is applied, which must be constant for all characterization testing. The size of the bonding site directly influences the strength of the resultant adhesive system bond, and inadvertent variations in the size of the test specimen bonding sites result in unreliable characterization databases. The other primary factor for consideration is that the fixture utilized to prepare test specimens must facilitate curing of the adhesive system being tested under conditions that approximate as closely as possible the curing conditions that will be encountered in actual production processes.
A number of adhesive systems are characterized by room temperature or oven curing (elevated temperatures) cycles at a constant cure pressure. Such adhesive systems may be readily evaluated to provide data for adhesive systems property databases using a simple, spring-loaded jig fixture JF of the type illustrated in FIG. 1. One or more test specimens, formed by overlapping pairs of coupons and interposing the adhesive system to be characterized in the bonding site defined by the overlap area of the coupons, are disposed in the jig fixture JF. The jig fixture JF is operative to apply a constant mechanical pressure over the bonding site. Such a jig fixture JF is generally satisfactory for room temperature or oven cured adhesive systems. However, since the test specimens are not totally secured by the jig fixture JF, bonding site variations may be induced in test specimens through careless handling of the jig fixture JF during the preparation process. Also, the jig fixture JF is per se limited to applying a single constant pressure to the bonding site during cure of the adhesive system.
With the advent of new production processes and/or new structural materials, for example, production processes involving composite materials, a need has emerged to provide standardized and reproducible characterization of adhesive systems used in such new production processes, and especially those that involve bonding of composite structural materials As discussed above, such characterization schemes should emulate as closely as possible the conditions inherent in the actual production processes so that adhesive systems may be characterized in terms of actual production applications.
For example, autoclave or Therm-X.TM. (see, e.g., U.S. Pat. Nos. 4,940,563 and 4,770,835) production processes are being more frequently utilized by industry, especially the aerospace industry, to fabricate composite structures. Such production processes require adhesive bonding of structural materials that are encapsulated in specialized sealed vacuum bagging films, which are then evacuated so that the vacuum bagging material contacts the structural material. The encapsulated materials are then inserted in a fluid-pressurized autoclave, and subjected to cure cycles that generally comprise variable fluid pressures and temperatures over time.
The prior art jig fixture JF described hereinabove, however, is only capable of applying a constant force over time, and, as such, is not suitable for use in autoclave-type curing processes. Moreover, the bulky structure of the prior art jig fixture JF is not readily amenable to encapsulation within vacuum bagging material. Furthermore, the complex configuration of the prior art jig fixture JF may cause damage to the vacuum bag material during handling and/or cure processing.
Industry has endeavored to develop a fixture for producing test specimens that is compatible with the autoclave-type cure environment to produce standardized test specimens for adhesive systems characterization testing. Test specimens, which comprise overlapped standardized coupons with the adhesive system to be tested applied thereto, have been assembled on a flat base plate fixture, encapsulated in vacuum bag material, evacuated, and cured in an autoclave-type environment Test specimens produced in this manner, however, were found to have been subjected to skewing and/or wedging effects during handling and/or the curing process that resulted in marked variations in bonding site areas, i.e., the test specimens were not usable for adhesive systems characterization testing.
To prevent skewing and/or wedging effects, test specimens were taped to the base plate fixture Such test specimens were still found to have cure process and/or handling induced variations in bonding site areas. Pencil marks were used to define test specimen bonding site areas in the hopes of producing test specimens having constant area bonding sites It was discovered, however, that the pencil lead was inducing premature fractures in the adhesive bonds. Attempts were made to define the bonding sites using masking tape. The masking tape, however, was found to introduce entrapped air in the adhesive bonds.
A need exists for an adhesive systems test specimen fixture that can be used in an autoclave-type environment and which will produce test specimens having constant area bonding sites, which may then be used for characterization testing of adhesive systems. Such a fixture should be operative to precisely position pairs of standardized coupons in overlapped relation to demarcate standardized bonding sites, i.e., constant area bonding sites. Such a fixture should also include means to secure precisely positioned test specimens to prevent skewing and/or wedging of test specimens during handling and/or cure processing. The fixture may have a configuration which facilitates envelopment of the test specimens within vacuum bagging material rather than encapsulating the entire fixture.
Preferably the fixture may also include an integral means for interconnecting the fixture, rather than the vacuum bagging material, directly to a vacuum source. Generally, prior art autoclave fixturing assemblies utilized a portable vacuum source port connector that was connected directly to the vacuum bagging material. Such a connection tended to induce tears in the vacuum bagging material.